This invention relates to a shift control apparatus used in an automatic transmission for a vehicle having at least one frictional engaging element and an electromagnetic valve to control the hydraulic pressure supplied to the frictional engaging element and achieving a plurality of speed ratios including a predetermined speed ratio achieved by engaging the frictional engaging element, to reduce speed shift shocks during a down shift in association with retardation of the vehicle.
The automatic transmission for a vehicle selectively supplies hydraulic fluid to frictional engaging elements such as clutches and breaks to connect a desired rotatory element in its gear system to an input shaft of the transmission or fix the element to a transmission casing, thereby automatically changing the speed ratio according to operation conditions of the vehicle.
Such an automatic transmission for a vehicle is required to be small in speed-shift shocks to protect various parts and components and maintain comfortable drive feeling. For this purpose, an automatic transmission for a vehicle has been proposed which uses a proper electronic control over the hydraulic pressure and its supply timing to frictional engaging elements, aiming for reduced speed-shift shocks.
An example of the structure of such an automatic transmission for a vehicle is shown in FIG. 6.
Referring to FIG. 6, a crack shaft 12 of an engine 11 is integrally connected with an impeller 14 of a torque converted 13. The torque converter has the impeller 14, a turbine 15, a stator 16, and a one-way clutch 17. The stator 16 is connected to a transmission case 18 by the one-way clutch 17. By the function of the one-way clutch, the stator is allowed to rotate in the same direction as the crank shaft 12 but is not allowed to rotate in the reverse direction. The torque transmitted to the turbine 15 is transmitted to the input shaft 19 (hereinafter referred to as the "transmission input shaft") of the gear transmission apparatus to achieve four forward speeds and a single reverse speed disposed at the rear of the torque converter 13.
The gear transmission apparatus comprises three clutches 20, 21, and 22, two breaks 23 and 24, one one-way clutch 25, and one ravigneaux type planetary gear mechanism 26. The ravigneaux type planetary gear mechanism 26 comprises a ring gear 27, a long pinion gear 28, a short pinion gear 29, a front sun gear 30, a rear sun gear 31, and a carried 32. The carrier 32 rotatably supports the pinion gears 28 and 29 and is rotatably engaged with the transmission input shaft 19.
The ring gear 27 connected to a transmission output shaft 33. The front sun gear 30 is connected to the transmission input shaft 19 through a kickdown drum 34 and a front clutch 20. Furthermore, the rear sun gear 31 is connected to the transmission input shaft 19 through a rear clutch 21. The carrier 32 is connected to a transmission casing 18 through a low reverse break 24 and the one-way clutch 25 and to the transmission input shaft 19 through a 4th-speed clutch 22 disposed at the rear end of the gear transmission apparatus. The kickdown drum 34 is integrally connectable to the transmission casing 18 by a kickdown break 23. Torque passed through the ravigneaux type planetary gear mechanism 26 is transmitted from a drive gear 35 mounted to the transmission output shaft 33 to the drive shaft side of driving wheels (not shown).
The clutches 20 to 22 and the breaks 23 and 24 as frictional engaging elements individually comprises hydraulic mechanisms provided with engaging piston devices or servo mechanisms. These hydraulic mechanisms are operated through a hydraulic control unit (not shown) by hydraulic fluid generated by an oil pump 36 connected to the impeller 14 of the torque converted 13.
Detailed structure an functions of the mechanisms are already known, for example, in Japanese Patent Publication Laid-open 58-46248/1983, 58-54270/1983, or 61-31749/1986. Thus, selective engagement of various frictional engaging elements is achieved according to the position of a shift lever provided beside the driver's seat of the vehicle (not shown) selected by the driver and operation conditions of the vehicle, and various speed ratios are automatically achieved through the hydraulic control unit according to instructions from an electronic control unit to control the operation conditions of the engine 11.
The select pattern of the shift lever includes P (parking), R (reverse), N (neutral), D (automatic three forward speeds or automatic four forward speeds), 2 (automatic two forward speeds), and L (fixed to the 1st speed) positions. With the shift lever set to the D position, when an auxiliary switch (over-drive switch, not shown) is operated, the automatic three forward speeds or the automatic four forward speeds can be selected. The functions of the individual frictional engaging elements when the shift lever is set to the individual positions are shown in FIG. 7. In the figure, symbol " " indicates that the engaging is achieved only when the L position is selected.
In the past, when the making an automatic down-shift in association with retardation of a vehicle equipped with such an automatic transmission, a speed ratio according to the current vehicle speed and the throttle opening has been read from a graph which is predetermined according to the vehicle speed and the throttle opening representing the load to the engine 11. When the current speed ratio differs from a speed ratio read from the graph, a hydraulic control valve is driven at a predetermined duty ratio to supply a predetermined engaging hydraulic pressure to the engaging side frictional engaging element, in order to achieve the newly read speed ratio.
In a prior art automatic transmission for a vehicle for a vehicle shown in FIG. 6 and FIG. 7 in which a plurality of speed ratios are achieved by electronically controlling hydraulic pressures to a plurality of frictional engaging elements to selectively engage these frictional engaging elements, the hydraulic control value is driven at a predetermined duty ratio so that a predetermined hydraulic pressure is supplied to the engaging side frictional engaging element independent of the retardation rate of the vehicle.
As a result, there occurs a difference in hydraulic pressure supplied to the engaging side frictional engaging element between a rapid retardation when the break pedal is rapidly pressed down and a moderate retardation when the bake pedal is not pressed down.
For example, when control is set so that speed-shift shocks are reduced at a down-shift in association with a moderate retardation of the vehicle, the hydraulic pressure supplied to the engaging side frictional engaging element tends to become too low because the engine speed rapidly decreases at a down-shift in association with rapid braking of the vehicle. As a result, large speed-shift shocks tend to occur due to a delay in engaging the engagingr142e frictional engaging element.
On the contrary, when the control is set so that speed-shift shocks are reduced at a down-shift in association with a rapid retardation of the vehicle, a high hydraulic pressure is supplied to the engaging side frictional engaging element at a down-shift in association with a moderate retardation of the vehicle where almost no decrease occurs in the engine speed. As a result, engaging of the engaging side frictional engaging elements tends to be too early, resulting in high speed-shift shocks.